This invention relates to a target for use in a photoconductive image pickup tube and more particularly to a P-type photoconductive film having a novel composition.
In recent years, a target of the novel rectifying contact type for use in photoconductive image pickup tubes has been proposed wherein a heterojunction is formed between a P-type photoconductive film containing amorphous selinium, tellurium and arsenic and an N-type transparent conductive film. U.S. Pat. Nos. 4,007,473 and 4,007,395, both issued to Nonaka et al, dated Feb. 8, 1977 and assigned to the same assignee as the present application disclose a target for use in a photoconductive image pickup tube wherein the distribution of tellurium contained in the P-type photoconductive layer is spaced from the heterojunction and localized in the vicinity of the heterojunction. The targets disclosed in these patents are characterized in that the residual image and flare of the image are small, and that the targets have high resolution, less picture defect in the form of white spots and can be manufactured readily.
However in such targets, not only the sensitivity to the voltage impressed upon the target and the saturation characteristic are not sufficiently high, but also it has been impossible to suppress the dark current to a small value. As is well known in the art, when the sensitivity to the applied voltage and the saturation characteristic are poor, there are such difficulties that it is nessary to increase the voltage applied to the target as the saturation lags and that the sensitivity decreases for all incident light as well as blue light. Morecover, a large dark current results in a great deterioration of the picture quality.